For 8 years, the Honda Civic CNG has topped ACEEE's rankings of the greenest vehicles. See http://aceee.org/press/2012/02/electric-car-tops-greenest-vehicle-l. But this year it fell to a tie for second with the Nissan Leaf in ACEEE's 14th annual rankings. After a long reign, the Honda Civic CNG is no longer the greenest car, but it remains incredibly clean, when compared to all alternatives. It also costs about $2 less per gallon to operate than the gasoline Honda Civic, since CNG is typically selling for between $1.25 and $2.00 per gallon.
What did ACEEE rank as the greenest car? The Mistubishi i-MIEV battery electric vehicle. The Toyota Prius, Honda Insight, and the Smart ForTwo were 4th, 5th, and 6th in the rankings.
To be ranked, a car must have more than 1,000 sales. As with all rankings, criteria and assumptions play a role. The Chevy Volt was hurt by an assumption that it was driven 65% of miles traveled on electricity, with the remainder on gasoline. A further assumption for electric cars was that electricity was generated by a national average mix of power plant resources.